Polish by rubbing (7)
I believe the answer is:
burnish
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">polish by rubbing</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for burnish that I've seen before include "Rub to shiny finish" , "Polish to a high shine" , "Make shiny by friction" , "Polish by rubbing" , "Buff up" .)